(a) Field of the Invention
Broadly speaking, this invention relates to subterranean roadway supports. More particularly, in a preferred embodiment, this invention relates to an improved stilt for receiving an I-beam joist support.
(b) Discussion of the Prior Art
During the mining of subterranean minerals, it is frequently necessary to construct roads or railways from the mine entrance to the point of mineral production. The roads may be driven within the mineral vein itself or in the strata above or below the vein. Where the vein is not of the required height, additional elevation may be obtained by excavating some of the roof or floor strata.
The extraction of the mineral or the excavation of the surrounding strata during construction of the road, will, in general, disturb the equilibrium of the stresses in the surrounding strata. The supports which are typically erected to prevent the collapse of the roof, or lateral movement of the sides of the road, must therefore incorporate in their design components to allow for the initial rapid strata movement--both vertical and lateral--which typically occurs while the region is consolidating itself.
Ideally, the aforementioned components continue to take up strata movement throughout the consolidation period such that at the end of this period the foot of the support leg will be touching the floor, thus offering maximum resistance to any further strata movement.
The form taken by these supports typically comprises at least two vertical, or nearly vertical, legs, usually a rolled steel I-beam joist, which hold a roof beam in contact with the roof of the mine road. Alternatively, the supports may comprise two or more curved I-beams which are coupled together in an arch profile, the lower extremities of the arch acting as legs, while the upper curve of the arch contacts the roof of the road.
The components which are attached to the support legs to control the rate of yield are known in the industry as stilts. One particular form of stilt comprises a rectangular steel tube, the dimensions of which are chosen to allow the support leg to enter therein. The lower section of the tube is provided with longitudinal indentations on opposing faces of the tube. The indentations protrude into the tube with their upper face being angled to allow the leg of the support to enter the stilt in a gradual manner. Vertical strata pressure forces the support leg downward deforming the indentations and thereby absorbing a certan amount of the strata pressure and so controlling the yielding support legs.
In order to resist lateral strata movement, it is usual to erect the support legs such that they are inclined at the top, i.e. into the roadway, the angle of this inclination being 5 degrees from the vertical. In addition, if the floor of the roadway is soft, the stilt will tend to penetrate the floor when the support is under load from the overlaying strata, before the stilt reaches its designed yield load. In such conditions, it is usual to attach a base plate to the support which has an area of floor contact which is sufficient to prevent this penetration from happening.
Such stilts and base plates are well known in the mining industry but suffer from several disadvantages. More specifically, in the prior art devices: (1) The flanges of the I-beam support legs must come into contact with the aforementioned indentations if the stilt is to perform its designed function, but, if by error the support leg is placed into the stilt the wrong way round, i.e. rotated through an angle of 90 degrees, the support leg will not make the necessary contact with the stilt indentations and is, thus, rendered ineffective; (2) The upper section of the tube has no yielding indentations and is thus able to accept the insertion of the support leg; however, the length of this entry section is critical in that it provides the required rigidity to prevent lateral displacement of either the stilt or the support leg before yielding commences and the prior art devices do not satisfy this criticality; (3) the angle formed by the upper receiving face of the indentations is likewise critical; if the angle is too large, it will cause the initial yield rate to be erratic, with consequent load shedding of the supports with possible damage to a friable roof strata. On the other hand, if the abovementioned angle is too small, the initial rate of yield will be so rapid that valuable yielding distance will be lost, plus damage to the mine roadway roof.